1. Field of the Invention
The present invention relates to a method and to an apparatus for localizing weak points within an electrical circuit in which the circuit to be tested is covered with a liquid crystal and is heated to a temperature just below the clearing point of the liquid crystal, and in which the liquid crystal converts into its unordered condition given a temperature rise. The method is particularly characterized in that at least one current is produced by irradiating a three-dimensional region by generating electron-hole pairs within the electrical circuit, the at least one current causing a temperature rise at at least one weak point of the electrical circuit. The apparatus comprises a means for heating the electrical circuit comprising a liquid crystal on the surface of the electrical circuit and is particularly characterized by means for irradiating the three-dimensional region within the electrical circuit and for generating electron-hole pairs in the three-dimensional region.
2. Description of the Prior Art
Thermography, i.e. the measurement of temperature distributions on surfaces, is increasingly gaining in significance for the development and quality analysis of integrated circuits or of other current-permeated electrical components. The presently most sensitive method for identifying "hot spots" (locally limited thermally loaded regions) is temperature measurement with liquid crystal layers in a polarization microscope. The liquid crystal layers are applied to the surfaces to be investigated. The study of birefringence of the liquid crystal serves in the polarized light for the identification of structures of this liquid crystal. Conclusions can be drawn regarding temperature differences within the surface to be investigated from structural changes of the liquid crystal layer on the surface to be investigated. Electrical properties within this surface can then be concluded from the temperature differences within the surface to be investigated.
When a liquid crystal layer has been applied to the surface of an integrated circuit, the electrical property (dissipated power, temperature) of that location on the surface which is impinged by a laser beam can be concluded from the intensity of the reflection of a laser beam which occurs through the system liquid crystal layer/surface.
The principle of a "hot spot" detector is disclosed in the PCT application WO 83/01 989. This application is based on a U.S. application, Ser. No. 326,224, filed Dec. 1, 1981 now U.S. Pat. No. 4,466,746, and fully incorporated herein by this reference.
Only the hottest locations can be detected with such a known thermography method. A dosing of the temperature elevation for the detection of weak points which are more or less pronounced is not possible with such a known thermography method. A localization of leakage current channels with this known thermography method is therefore possible practically only in formulations.
As described in any textbook of scanning electron microscopy, a pn junction of a semiconductor diode can be imaged in a scanning electron microscope with the assistance of an induced specimen current (EBIC: Electron Beam Induced Current). Primary electrons thereby penetrate into the semiconductor diode with an energy of, for example, 20 keV. These primary electrons are thereby scattered and loose their energy in a so-called scatter volume. The primary electrons generate electron-hole pairs in this scatter volume by ionization. When the scatter volume is located at a sufficiently large distance from the pn junction of the semiconductor diode, then the electrons and holes recombine with one another after they have traversed, on average, a path corresponding to their respective diffusion lengths. When the primary electron beam migrates across the surface of the semiconductor diode and thereby approaches the pn junction, then diffusing charge carriers finally proceed into the influencing region of the electrical field of the space charge zone of the pn junction. The respective minority carriers are respectively accelerated onto the other side of the pn junction by this electrical field (electrons on the n side, holes on the p side). Given a closed external circuit, a "charge separation current" (EBIC) is present which can be measured, amplified and used, for example, for the intensity modulation of the picture tube of the scanning electron microscope.
A closed circuit which encompasses the leakage current channel to be localized is required for generating such a charge separation circuit (EBIC). Such a closed circuit at the leakage current, however, is generally not present within integrated circuits.